Various types of optical devices have been proposed. For example, as shown in FIG. 3A, U.S. Pat. No. 4,082,434 discloses a wide-angle optical system for a door viewer comprising a concave objective lens 21, an intermediate concave lens 22 and a convex eyepiece lens 23 which is positioned at a predetermined distance from the objective lens 21. The aberration of the erect virtual image derived from the objective lens 21 is corrected by the intermediate lens 22, the image from the intermediate lens 22 is magnified by the eyepiece lens 23 and thus the magnified final erect virtual image is formed on the eyepiece lens 23.
On the other hand, as shown in FIG. 3B, U.S. Pat. No. 4,257,670 disclosed an optical peephole device which is constituted by an optical system composed of three lens assemblies disposed serially along a common optical axis. The first assembly has a doublet constituted by a thick-edged meniscus 31 and a double-concave lens 32, the second assembly has five identical plano-convex lens 33, 34, 35, 36, and 37 that are equidistantly spaced from each other, and the third assembly provides accommodation and is made of a plano-convex lens 38 and an eyepiece 61. In this device, an erect virtual image from the meniscus 31 becomes an inverted real image through the plano-convex lens 33, other plano-convex lenses 34 through 38 correct the aberration and invert again the inverted real image, and thus the final erect real image is formed on the plano-convex lens 38.
However, the former has defects in that as a virtual image is cast on the eyepiece lens, it cannot be made visible on an image formation means such as a ground glass, and therefore the user's eye must be close to the eyepiece lens to view an outside caller or object. Further, because of the small effective diameter of the concave objective lens, the image formed on the eyepiece lens becomes dark due to small amount of incident light. Moreover, when the effective diameter of the concave objective lens is increased in order to brighten the dark image, the caller can view the inside of the door from outside.
The latter device also had disadvantages that though the image derived from the plano-convex lens is erect and real, the luminosity of the formed image is poor due to a large loss of light through the several lenses, no image formation means such as the ground glass can be employed and the user's eye must be close to the eyepiece. Furthermore, the latter device is not suitable for being mounted in a door but suitable for being inserted through a peephole in the thick wall of a chamber because of its large overall length.
In related U.S. Pat. No. 4,892,399 to Ahn, a door viewer projects an image on a ground glass screen. However additional efforts have been made to simplify and reduce the cost of such a door viewer, while still providing a clear cast image on the ground glass screen.